S. cerevisiae

So now I'm curious and I suspect you might know. When yeast metabolize ethanol in the presence of oxygen, what do they produce and does it affect flavor?

Yeast cells basically reverse the acetaldehyde to ethanol process.

The following reduction occurs:

Ethanol -> Acetaldehyde -> Acetate -> Acetyl-CoA

Acetyl-CoA is used as an energy source via the Krebs Cycle (a.k.a. the TCA Cycle). As with glycolysis, incomplete utilization of ethanol as a carbon source can result in acetaldehyde and/or ester production.

Like so many things, we are working with sub optimal equipment, knowledge, and facilities in the homebrew world

Commercial brewing is the bastardization of homebrewing, not the other way around. I really feel that they are the ones in the sub-optimal position.

^^THIS^^

I appreciate and absorb every oz of info that I can get my hands on from commercial brewing. Then I filter it to determine if it's something I can use at home, in some fashion. Sometimes yes, sometimes no. But I don't feel I've failed as a brewer if I can't. It just didn't work for me.

S. cerevisiae

I don't reuse yeast very often, but when I do I follow the procedure outlined in Yeast.

Is there a bettter method I should investigate?

The best way to crop is to "top crop" at high krausen. However, top-cropping requires one to use a true top-cropping strain. Top-cropping naturally purifies a culture because wild yeast and bacteria do not floc to the top. Top-cropped yeast can be repitched almost indefinitely.

When using a non-top-cropping yeast strain, I usually leave enough liquid behind after racking to be able to swirl the solids back into suspension (my primary volume is 1/3 to 1/2 gallon larger than the volume I expect to rack). Swirling the solids back into solution using green beer, waiting a few minutes for the heaviest fraction to settle, and then decanting the liquid fraction has the same effect as rinsing with boiled water; however, it keeps the low pH, ethanol laden environment intact. If one wants to attempt to rid the culture of mutants, one can perform a second decant as soon as a creamy layer of yeast forms in the first decant.

One of the first things that a yeast culture does when pitched into a batch of wort is to lower the pH from around 5.2 to around 4.1. One has heard that pathogens do not grow in beer. One of the reasons why pathogens do not grow in beer is due to its relatively low pH. Clostridium botulin growth is inhibited below pH 4.6.

Contrary to what was written in early amateur brewing books such as Brewing Lager Beer, brewing yeast cultures do not respire in wort due to a phenomenon known as the Crabtree effect. Hence, brewing yeasts do not go through a period of resparitive (aerobic) growth before they start to reproduce fermentatively (anaerobic growth). In the presence of glucose levels above the Crabtree threshold, all reproduction is fermentative. As you probably noticed while reading Yeast, yeast cultures use dissolved oxygen to build ergosterol and unsaturated fatty acid (UFA) reserves (these reserves are shared with with all of the daughter cells). Yeast perform this feat by shunting oxygen to the respirative (aerobic) metabolic pathway while simultaneously metabolizing the carbon source via the fermentative (anaerobic) metabolic pathway.

What this preference to reproduce fermentatively means to a brewer is that yeast cells pretty much start producing ethanol as soon as they are pitched into a batch of wort. While ethanol has a limiting effect on the viability of a yeast culture, it also protects the culture from infection. Boiled water is not truly sterile. Boiled tap water also tends to have a pH of at least 7.0; therefore, it raises the pH of the culture.

With the above said, most experienced amateur brewers eventually reach the conclusion that one can just crop and repitch without doing anything to separate the viable cells from the dead cells and break material, especially if they leave most of the break and hop material in the kettle. Less is definitely more when cropping yeast.